This invention relates to a method of inspecting a hole in a component, and to a device for use in such a method. The invention is particularly, although not exclusively, concerned with the in situ inspection of holes in components of gas turbine engines.
Many components of gas turbine engines, such as turbine vanes and blades, and combustor components, are exposed to very high temperatures in operation of the engine, which can be close to or exceed the melting point of the material from which the component is made. It is well known to cool such components by means of high pressure air taken from the compressor of the engine and ducted to the component. For this purpose, turbine blades and vanes may have internal cavities to which the cooling air is supplied, and holes which extend from the cavity to the outside surface of the blade through which the cooling air can pass to rejoin the main gas flow through the engine. The cooling air not only extracts heat from the blade or vane as it passes through the cavity and the cooling holes, but also forms a film of cooler air over the surface of the blade, shielding it from the hot gas flow.
Holes are disposed in an array on the surface of the vane or blade, and the disposition of the holes in the array is carefully determined in order to provide the maximum cooling effect with minimum use of cooling air. If a hole becomes blocked, for example by debris entering the cooling air supply, the temperature of the surrounding surface of the blade or vane will increase. If enough holes become blocked, the resulting temperature increase can lead to failure of the material of the blade or vane, which can result in failure of the component itself, and possibly the entire engine. It is therefore desirable to inspect cooling holes in such components, and particularly aerofoil components such as turbine blades and vanes, to check that the cooling holes remain sufficiently clear so that they can perform their intended cooling function.
Various cooling hole inspection techniques have been proposed, for example in U.S. Pat. No. 4,644,162, U.S. Pat. No. 5,111,046 and U.S. Pat. No. 6,524,395. The known techniques all require the inspected component to be removed from the engine. Strip down of a gas turbine engine to the extent required to remove turbine blades and vanes is very time consuming and expensive, and consequently extensive cooling hole inspection is not currently practical between major engine overhauls.